Rashi (15:20) writes that challah is called terumah because like terumah it has no shiur -- any amount (m'doraysa) may be given to the kohen to fulfill the mitzvah. Yet, one pasuk later, Rashi comments that the word "titnu" teaches that one must give a not-insignificant amount to the kohen so that it counts as a proper nesina, an act of giving. Is there a minimum amount required to fulfill challah or isn't there?
The Noda b'Yehuda answers (as we've discussed before, and here as well) that there are two aspects to the mitzvah of challah: 1) removing the status of tevel from the dough; 2) making a gift to the kohen. Seperating any amount of dough removes the status of tevel from the dough. However, to fulfill the mitzvah of nesina, of presenting a gift to the kohen, requires a more significant gift.